1. Field of the Invention
The present invention generally relates to a reticle with a polarizer. In particular, the present invention relates to a reticle, i.e. a photo mask with one or more polarizers to selectively filter illumination light, that is, the light from the illumination source of a photo scanner illuminator when the illumination light passes through the polarizer in a semiconductor photolithography process. The reticle of the present invention allows the individual optimization of the polarization state of the light for each pattern on a same reticle to be processed without employing a special illuminator to optimize the illumination source shape locally for those patterns.
2. Description of the Prior Art
As the integration of ICs increases, the critical dimension of semiconductors becomes smaller. Therefore, it is desirable to increase the resolution limit of optical exposure tools. A conventional method for improving resolution includes the steps of: off-axis illumination, immersion lithography and increasing the numerical aperture of the lens. Applying polarization to the source illumination can further improve the imaging contrast. Different parts of the source illumination can use different polarization states.
In general, a photo mask, or called a reticle, is composed of a mask substrate, a opaque patterned layer, such as Chrome (Cr), and/or a partially transmissive patterned layer, such as molybdenum silicide (MoSi). The mask substrate can be a quartz substrate, and the patterned layer(s) covers the quartz substrate and has a pattern to be transferred to the wafer. The illumination source can be polarized into two modes: transverse-electric (TE) mode and transverse-magnetic (TM) mode.
Generally speaking, polarized light with high NA (numerical aperture) and an off-axis illumination (like dipole one) is employed to transfer very small repeating features onto a wafer. For example, light of 193 nm wavelength and immersion scanners use polarized light to improve image log-slope (ILS) at high NA. With both TM & TE components in the light, when the light incident angle becomes bigger, the contrast of a line/space pattern image from the TM fraction of the light drops drastically.
In the semiconductor field, in order to obtain the best imaging results for each individual pattern on a mask, some special illuminators may be used to optimize the illuminator's source shape of light for all patterns. Although this is the standard industrial approach, the introduction of the special illuminators usually leads to a trade-off in best imaging of individual pattern.
Further, the introduction of the special illuminator also has some negative influence on the production cost. Accordingly, another approach is still needed to obtain the best imaging results for each individual pattern without the adverse consequence encountered in the conventional methods.